Process for the preparation and recovery of carbon dioxide from waste gas or fumes produced by combustible oxidation

ABSTRACT

A process for the separation and recovery of carbon dioxide from waste gases produced by combustible oxidation is described comprising the steps of feeding a flow of waste gas to a gas semipermeable material, separating a gaseous flow comprising high concentrated carbon dioxide from said flow of waste gas through said gas semipermeable material, and employing at least a portion of said gaseous flow comprising high concentrated carbon dioxide as feed raw material in an industrial production plant and/or stockpiling at least a portion of said gaseous flow comprising carbon dioxide.

FIELD OF APPLICATION

[0001] In its most general aspect, the present invention relates to aprocess for the separation and recovery of carbon dioxide from waste gasor fumes produced by combustible oxidation.

[0002] In particular, the present invention relates to a process for theseparation and recovery of carbon dioxide from exhaust gases or fumesproduced by the oxidation of fossil fuels or fractions and derivativesthereof with air.

[0003] The term “oxidation” is meant to comprise both the normalcombustion of fuels, particularly fossil fuels, with air, carried out ona domestic or industrial scale, and the electrochemical oxidationthereof, occurring, for example, in the fuel cells.

PRIOR ART

[0004] It is well known that exhaust (waste) gases or fumes produced bythe combustion or by other oxidative processes of fossil fuels(combustible) in industrial or domestic plants, are dispersed in theatmosphere causing various environmental impact problems. The mostsevere of such problems relates to the overall heating of the planet,known as “greenhouse effect” for which the carbon dioxide contained insaid exhaust gases or fumes is responsible.

[0005] Furthermore, it is known that carbon dioxide is a feed rawmaterial in several industrial processes; for carrying out suchprocesses, the thermal energy produced by the combustion of fossil fuelsis usually employed. For these processes, therefore, it could beconvenient to separate and recover at least part of the carbon dioxidefrom the combustion exhaust gases, in order to increase the productioncapacity and/or to reduce the purchasing costs of this raw material.

[0006] For example, in the processes for the production of ammonia andurea or methanol, it is known that the feed raw materials such ashydrogen, carbon monoxide and carbon dioxide are generally obtained inthe form of a gaseous mixture through the reforming of methane or otherlight hydrocarbons such as natural gas, LPG (liquefied petroleum gas),naphtha.

[0007] The methane conversion is carried out in a dedicated furnace of areforming plant, usually associated to the that used for the productionof ammonia and urea or methanol, exploiting the thermal energy producedby the combustion of a part of the feed methane with air.

[0008] Within the production process of ammonia and urea, nitrogen isthen added in stoichiometric amount to the gaseous mixture obtainedthrough the reforming, in order to convert hydrogen into ammonia.

[0009] However, in this way, the amount of carbon dioxide contained inthe aforesaid gaseous mixture is smaller than the stoichiometric amountrequired to convert into urea all the ammonia produced, so that the ureaproduction plant capacity is disadvantageously reduced.

[0010] Instead, in the methanol production process, it is the hydrogenof the gaseous mixture obtained through the reforming that is in excesswith respect to the amount necessary for converting all the carbonmonoxide and carbon dioxide into methanol and therefore part of it ispurged from the synthesis reactor and often used as fuel.

[0011] In the above mentioned two processes, hence, it is clear that theproduction capacity of urea and methanol, respectively, could besignificantly increased, should it be possible to recover even only partof the carbon dioxide contained in the combustion gases of the methanereforming plant.

[0012] Therefore, the problem regarding the separation and recovery ofcarbon dioxide from the combustion gases or fumes is quite felt and inthe last decades has been the subject of several studies.

[0013] The larger part of these studies was directed to the so-called“wet” separation and recovery processes of carbon dioxide. That is tosay, processes based upon the scrubbing of the combustion gases withsuitable solutions or solvents able to adsorb selectively carbon dioxideand the recovery of the adsorbed carbon dioxide through heating of theadsorbing solution or solvent.

[0014] Various processes of the aforesaid type have been developed inthe prior art; however, they suffer of various drawbacks that limittheir industrial applicability.

[0015] One of the drawbacks, which are more often encountered lies inthe fact that the scrubbing solution is subjected to oxidation phenomenaof its components, which phenomena are due to the presence of oxygen inthe combustion gases, and therefore requires a frequent replacement.

[0016] Furthermore, it shall be noted that the combustion gases usuallycontain also sulphur and nitric oxides (SOx and NOx), that react withsome components of the scrubbing solution, creating stable salts andother harmful compounds of difficult removal and disposal.

[0017] Hence, the need of frequently replacing the scrubbing solution aswell the removal and disposal of the harmful compounds deriving from itsdegradation imply relevant operating costs for the aforesaid knownprocesses of gaseous carbon dioxide separation and recovery.

[0018] Furthermore, the equipment required for implementing the carbondioxide separation and recovery processes of the prior art arecomplicated, expensive, difficult to be operated and have a large size,and therefore high investment and maintenance costs are also implied.

[0019] The technical problem underlying the present invention is that ofproviding a process for the separation and recovery of carbon dioxidefrom waste gases produced by combustible oxidation that is simple andcost-effective to be carried out, and does not exhibit the previouslydescribed drawbacks with reference to the carbon dioxide separation andrecovery processes of the prior art.

SUMMARY OF THE INVENTION

[0020] This technical problem is solved by a process for the separationand recovery of carbon dioxide from waste gases produced by combustibleoxidation comprising the steps of:

[0021] feeding a flow of waste gas to a gas semipermeable material,

[0022] separating a gaseous flow comprising high concentrated carbondioxide from said flow of waste gas through said gas semipermeablematerial, and

[0023] employing at least a portion of said gaseous flow comprising highconcentrated carbon dioxide as feed raw material in an industrialproduction plant and/or stockpiling at least a portion of said gaseousflow comprising carbon dioxide.

[0024] The gas semipermeable material can be chosen from the groupcomprising hollow fibre membranes and materials able to adsorbpreferentially carbon dioxide, such as the molecular sieves.

[0025] The hollow fibre membranes can be of the type preferentiallypermeable to carbon dioxide or of the type substantially non-permeableto this gas.

[0026] The term “molecular sieves” is meant to comprise all thoseconventional materials having micropores adapted to adsorbpreferentially the carbon dioxide contained in a gaseous mixture,including activated carbon. According to the specific way the carbondioxide is adsorbed and released, these materials are classified asmolecular sieves or activated carbons of the PSA (pressure swingadsorption) or TSA (temperature swing adsorption) type.

[0027] In the PSA adsorption process, the gas mixture containing carbondioxide is made pass through the molecular sieve under pressure in sucha way to promote the preferential adsorption of carbon dioxide in themicropores. Then, the pressure is reduced in such a way as to obtain adesorption of the carbon dioxide together with other gaseous componentspossibly retained therewith and accordingly regenerate the molecularsieve.

[0028] Differently, in the TSA method, the preferential adsorption ofcarbon dioxide into the micropores is carried out by letting the gaseousmixture containing carbon dioxide to be separated, pass through theabove mentioned molecular sieve, at a temperature not higher than 80° C.Then the temperature is increased, for example with the aid of a vapourflow, in such a way as to obtain a desorption of the carbon dioxidetogether with other gaseous components possibly retained therewith andaccordingly regenerate the molecular sieve.

[0029] Preferably, in the process according to the present invention atleast a molecular sieve of the TSA type is used.

[0030] The use of molecular sieves of the TSA type in the processaccording to the invention does not require the compression of largeamounts of gas to be separated and therefore is advantageous because ofthe resulting low energy costs.

[0031] Furthermore, in order to regenerate the molecular sieves of TPAtype, it is enough to let a vapour flow or, alternatively, a portion ofthe gas flow comprising high concentrated carbon dioxide, pass throughthese sieves, wherein such portion of gas flow is suitably heated at theregeneration temperature of such sieves.

[0032] Otherwise, the use of hollow fibre membrane or molecular sievesof PSA type in the process according to the invention is lessadvantageous, if compared with the use of molecular sieves of TSA type,because of the relevant energy costs connected to the requiredcompression of the exhaust gases to be treated.

[0033] Further on, the hollow fibre membranes are very expensive even ifthey guarantee a greater effectiveness and separation yield of thecarbon dioxide from other gaseous components contained in the combustionexhaust gases.

[0034] According to a preferred embodiment of the present invention, thegas semipermeable material is able to adsorb preferentially carbondioxide and the separation of the gaseous flow comprising highconcentrated carbon dioxide from said waste gas flow comprises the stepsof:

[0035] letting a waste gas flow permeate into said gas semipermeablematerial in such a way as to adsorb at least a relevant portion of thecarbon dioxide contained in said waste gas flow and obtain a permeatedgas flow with a low carbon dioxide content,

[0036] dispersing said permeated gas flow with low carbon dioxidecontent, and

[0037] deabsorbing said at least one relevant portion of carbon dioxidefrom said gas semipermeable material, thus obtaining said gaseous flowcomprising high concentrates of carbon dioxide.

[0038] The features and advantages of the process for the recovery ofcarbon dioxide from combustion exhaust gases according to the presentinvention will become clearer from the following description of anindicative and non-limiting example of implementation thereof, made withreference to the attached drawing.

BRIEF DESCRIPTION OF THE DRAWING

[0039]FIG. 1 shows a block diagram of an embodiment of implementation ofthe process for the separation and recovery of carbon dioxide from acombustion exhaust gas according to the present invention.

DETAILED DESCRIPTION

[0040] With reference to the annexed figure, block 1 refers to anequipment of a domestic or industrial plant for the combustion of afuel, in particular a fossil fuel, with air.

[0041] Block 2 refers to a heat exchanger for cooling an exhaust gasflow at high temperature produced by the combustion within block 1.

[0042] The gaseous composition of this exhaust gas flow mainly comprisescarbon dioxide, water, oxygen and nitrogen and, to a limited extent,nitric and sulphur oxides (SOx and NOx).

[0043] Block 3 refers to a compression unit adapted to compress up to adesired pressure the exhaust gas flow cooled within block 2. Such block3 is optional and becomes particularly important when for the carbondioxide separation PSA type molecular sieves or hollow fibre membranesare used, since it is necessary to suitably compress the exhaust gas tobe treated.

[0044] If TSA type molecular sieves are used, the block 3 can be omittedor, alternatively, it may-consist of a simple fan.

[0045] Block 4 refers to a gas semipermeable material, such as amembrane or a molecular sieve, to separate the gaseous flow comprisinghigh concentrated carbon dioxide from the exhaust gas flow coming fromthe block 2 or block 3 as it will be explained later on in the presentdescription.

[0046] Block 5 refers to another compression unit adapted to compress agas flow comprising high concentrated carbon dioxide coming from theblock 4.

[0047] Block 6 refers to another heat exchanger adapted to heat aportion of gas flow comprising high concentrated carbon dioxide comingfrom the block 4.

[0048] The flow line 7 indicates an exhaust gas flow at high temperatureproduced by the combustion within block 1.

[0049] This exhaust gas flow is then fed to the block 2 where it iscooled down to a temperature comprised between 20° and 80° C.

[0050] The flow line 8 indicates the cooled gas flow coming from theblock 3. If the gas semipermeable material of block 4 consists of ahollow fibre membrane or by a PSA type molecular sieve, the exhaust gasflow 8 is firstly compressed in the block 3 at a pressure comprisedbetween 1 abs bar and 20 abs bar, and then fed, as indicated by the flowline 9, to the block 4.

[0051] On the contrary, if the gas semipermeable material of block 4consists of a TSA type molecular sieve, block 3 may be omitted andtherefore the exhaust gas flow 8 coming from the block 2 is directly fedto the block 4.

[0052] The gas semipermeable material of block 4 provides for theseparation of a gas flow comprising high concentrated carbon dioxidefrom the exhaust gas flow 8 or 9.

[0053] Preferably, this material consists of a TSA type molecular sievethat allows the preferential passage of nitrogen, adsorbing at the sametime the mixture gaseous components containing oxygen, i.e. mainlycarbon dioxide, water and oxygen.

[0054] Therefore, as indicated by the flow line 10, at the outlet of theblock 4 a gas flow is obtained comprising mainly nitrogen that isdispersed in the atmosphere.

[0055] In order to obtain a desorption of the, carbon dioxide and theother oxygenated compounds adsorbed in the block 4, an interruption ofthe exhaust gas flow 8 or 9 to the block 4 and a regeneration of thehollow fibre membrane or of the molecular sieve represented in the block4 is provided.

[0056] In the case of a hollow fibre membrane or a molecular sieve ofthe PSA type, the regeneration is carried out by decreasing the pressurein the block 4 (decompression) in such a way as to separate the carbondioxide adsorbed in such materials.

[0057] In the case of a TSA type molecular sieve, the regeneration iscarried out in a manner that will be explained later on in the presentdescription.

[0058] As indicated by flow line 11, from the regeneration step agaseous flow is thus obtained, which turns out to have a carbon dioxideconcentration higher than that in the exhaust gas flow 8 or 9. Also theconcentration in the gaseous flow 11 of the other gaseous componentsadsorbed in the block 4 is higher than the concentration of thesecomponents in the exhaust gas flow 8 or 9.

[0059] Then, the gaseous flow 11 comprising high concentrated carbondioxide can be used as a feed raw material in suitable industrialprocesses, directly or after having been further treated. Alternatively,the flow 11 can be liquefied or stockpiled in a suitable manner in orderto be subsequently used according to specific needs.

[0060] For instance, the gaseous flow 11 can be compressed in block 5 toa suitable pressure, and the so obtained gaseous flow indicated by flowline 12 can be directly utilized as feed raw material in a plant for theproduction of urea or methanol.

[0061] Anyway, should the complete or partial removal from the flow 11of gaseous components, such as oxygen and nitric or sulphur oxides (SOx,NOx) be necessary, then it is possible to arrange for the passage of thegaseous flow 11 under suitable operative conditions through one or moremembranes or molecular sieves and/or for the treatment of the flow 11with other types of separation systems.

[0062] In this case, the flow 11 completely or partially purified fromthe above mentioned gaseous components can be compressed in the block 5and used in a plant for the urea of methanol production as a feed rawmaterial.

[0063] In the present example, a flow portion 11 comprising highconcentrated carbon dioxide, indicated by the flow line 13, is heated inblock 6 and fed to block 4 through the flow line 14, in order toregenerate the TSA-type molecular sieve.

[0064] Alternatively, for the above-mentioned regeneration it ispossible to use water steam at high temperature.

[0065] The regeneration implies the desorption of the gaseouscomponents, and in particular of carbon dioxide retained into themicropores of the TSA-type molecular sieve, which are recovered in theflow 11.

[0066] Obviously a man skilled in the art can make a plurality ofmodifications to the process according to the invention in order tofulfill specific and peculiar requirements, all falling within the scopeof protection of the invention as defined in the following claims.

1. Process for the separation and recovery of carbon dioxide from wastegases produced by combustible oxidation comprising the steps of: feedinga flow of waste gas to a gas semipermeable material that is a molecularsieve or activated carbon of TSA type, separating a gaseous flowcomprising high concentrated carbon dioxide from said flow of waste gasthrough said gas-semipermeable material, and employing at least aportion of said gaseous flow comprising high concentrated carbon dioxideas feed raw material in an industrial production plant and/orstockpiling at least a portion of said gaseous flow comprising carbondioxide.
 2. Process according to claim 1, wherein said gas semipermeablematerial is able to adsorb preferentially carbon dioxide and theseparation of the gaseous flow comprising high concentrated carbondioxide from said waste gas flow comprises the steps of: letting a wastegas flow permeate into said gas semipermeable material in such a way toadsorb at least a relevant portion of the carbon dioxide containedwithin said waste gas flow and to obtain a permeated gas flow with lowcarbon dioxide content, dispersing said permeated gas flow with lowcarbon dioxide content, and deadsorbing said at least one relevantportion of carbon dioxide from said gas semipermeable material, thusobtaining said gaseous flow comprising high concentrated carbon dioxide.3. Process according to any of the preceding claims, wherein saidcombustible is a fossil fuel.
 4. Process according to any of thepreceding claims, wherein said plant for industrial production is aplant for the production of ammonia and urea or methanol.